Small on purpose
A microdisplay is a screen that is physically tiny, often smaller than a postage stamp, yet still carries a full image. It is not meant to be looked at directly. Instead, a lens or other optics magnify it so that it appears large and fills a good part of the viewer’s vision. Electronic viewfinders and many headsets work this way.
The design choice follows from geometry. A screen held close to the eye cannot be focused on, so optics are needed to make the image appear farther away. Those optics also make it appear bigger. Making the screen small keeps the whole device compact and light enough to hold against the face or wear.
Why the pixels must be so small
Magnification enlarges everything, including the gaps between pixels. If a screen has pixels you could just make out at arm’s length, magnifying it several times would make the grid obvious. To look smooth through the optics, the pixels must be far smaller than on a phone or monitor.
This is why close-to-eye designs push pixel density to very high levels. The relationship is easiest to see by comparing how pixel density depends on viewing distance for ordinary screens: the closer or more magnified the view, the finer the pixels need to be. For a magnified screen, the relevant quantity is how many pixels cover each degree of the viewer’s field of view. More pixels per degree means a sharper impression, and the total pixel count needed rises quickly when the field of view is wide.
Resolution claims for headsets, including the headset resolution discussed in coverage of virtual reality, are meaningful only alongside field of view and optics. A high pixel count spread over a wide view can still give fewer pixels per degree than a lower count in a narrow view.
How the panels are built
Several approaches are explored. The general theme is a very small, dense array of pixels, often with the driving electronics built into a silicon chip rather than on glass. That makes fine detail possible.
- Emissive types, such as microscopic organic light emitters, produce light directly in each pixel. They can offer deep blacks and fast changes, but brightness and lifetime at tiny sizes are design concerns.
- Light-modulating types use a bright light source and an array that controls it, such as small liquid crystals on a reflective surface. They can be bright, but need illumination optics.
- Experimental emitter types, such as the microLED displays that are discussed on their own page, aim for high brightness in a very small area, though manufacturing them at this scale is difficult.
Which is best depends on the purpose, and the market has not settled on one winner.
Brightness, efficiency and heat
Magnifying optics lose some light, and see-through designs lose more. The panel may therefore need to be brighter than its modest size suggests, while remaining small enough that heat can be dissipated. Peak brightness is only part of the picture, since levels that can be held steadily matter too, as explained under peak brightness. Heat close to the face is also a comfort issue.
Optical side effects
Because the optics sit between eye and panel, they bring their own artifacts.
- Screen-door effect. The visible gaps between pixels look like a fine mesh. Higher density and optical diffusion reduce it.
- Edge blur and color fringes. Lenses are rarely perfect across the whole field of view, and the image is often corrected in software, which costs processing.
- Sweet spot. The eye must be in the right position for the sharpest image.
- Focus distance. Optics fix the apparent distance of the image, which affects eye comfort over long sessions.
Viewfinders
Electronic viewfinders in cameras are a long-established use. They give a bright image that is not washed out by sunlight, show information such as exposure settings, and can display the scene as the camera will record it. The small screen must refresh quickly enough that movement looks natural and show enough detail to judge focus, which links to topics like refresh rate and motion blur. Delay between the scene and the displayed image can matter for fast action.
Headsets
Headsets place a microdisplay, or a pair of them, in front of each eye. Demands are higher here: a wide field of view, low delay between head movement and image updates, and long wear time. See-through designs add the optical challenges covered under augmented reality. Because specifications vary so much between devices, they should be read with care rather than compared on a single number.
What remains unsettled
- How small can pixels get while still being bright and long-lasting?
- Can very wide fields of view be combined with thin optics?
- What approach best balances efficiency, cost and image quality?
- How will high pixel counts be driven without excessive power and heat?
These are active engineering problems. The core idea, though, is simple and durable: the closer and bigger a screen is made to appear, the smaller its pixels must be.